Yeah, even if it's fanmade, it still must not be overpowered. Here's the official rulebook from Konami. Read it if you wanna make cards properly. Except when you're making it for fun.
If you don't wanna read the whole PDF file, here's the summary for monsters: Monsters with 1 to 4 stars can be summoned normally, while 5 and 6 stars need at least 1 Tribute (destroy a monster on your side to summon it), and 7 stars and above require 2 Tributes.
Also, "Rarity" should be based on the character you're making, like if you're making a common, weak monster, like Peashooter, it should be Common, while upgrades being Rare, and PvZCC powerful (not overpowered) plants being from Super Rare or Ultimate Rare. Example for Mile: Sonic = Common, Super Sonic = Rare.
If you have any questions about Yu-Gi-Oh! or card-making, feel free to ask me. And if you play Yu-Gi-Oh! and want to duel with me, register on Dueling Network, then go to the Profile Viewer and search for "EpicOtaku" to add me as friend,